What am I going to do? I guess I'll take a bath to calm down.
Wait a second... why is the water spilling out?! I think I get it now!
I figured it out! It's is because of volume, or the amount of space something takes up. Gold is more dense, and so it will not take up as much space as silver. The goldsmith was still able to make them weight the same, though.
You see, the amount of gold you gave him makes less water spill out than the crown! The goldsmith DID put in silver!